1. Field of the Invention
The present invention generally relates to heating, ventilating, and air conditioning systems (HVAC systems), and more specifically to a thermostat for such a system, wherein the thermostat identifies minimum or maximum tolerable outdoor temperatures for a given desired indoor temperature and/or identifies minimum or maximum achievable indoor temperatures for a given outdoor temperature.
2. Description of Related Art
Temperature conditioning equipment, such as gas or oil furnaces, electric heaters, air conditioners, and heat pumps are often used to control or modulate the temperature of a room or other area of a building. Although there are many ways of controlling such equipment, one common method involves cycling the equipment on and off as needed.
For example, the controller disclosed in U.S. Pat. No. 4,292,813 includes an off-time counter that determines the duration of the off cycle. The counter is incremented when the measured run-time of the equipment is less than a desired minimum and is decremented when the measured run-time is greater than a desired maximum. If the measured run-time is within a desired range, then the prior off-time is maintained.
As another example, U.S. Pat. No. 5,647,533 discloses a controller that varies the run-time of a heat pump. The speed of an indoor blower is adjusted whenever the operating run-time fraction of a heat cycle passes a predetermined threshold, unless the run-time fraction is sufficiently changed compared to a previous run-time fraction.
Regardless of the control scheme, many factors can reduce the heating or cooling effectiveness of temperature conditioning equipment. The equipment's heating or cooling effectiveness is a function of several factors, such as the equipment's designed capacity; the cleanliness of various heat exchangers and air filters; refrigerant charge; thermal load, which can vary with the outdoor temperature and the desired indoor temperature; and mechanical condition of a compressor or blower associated with the equipment.
To address diminished or otherwise insufficient capacity, some controllers include provisions for analyzing the condition of the equipment and its surroundings. For example, U.S. Pat. No. 4,574,871 discloses a diagnostic thermostat or monitor that measures run time for a heat pump compressor and compares expected run times for a particular outdoor temperature. An alarm signal identifies when the system fails to operate within expected parameters.
Another example of a diagnostic thermostat is disclosed in U.S. Pat. No. 4,685,615. Here, the thermostat senses the outdoor temperature and accumulates degree-days and run-time of a heating or cooling unit. The thermostat can calculate, store, and display energy used per degree-day for a given period to indicate the performance or efficiency of the building and its heating or cooling system.
U.S. Pat. No. 5,729,474 discloses another method of monitoring the effectiveness of a heating or cooling system. An efficiency value representing the system's ability to change temperature in a predetermined zone is repeatedly computed. Comparing a current efficiency value with a previous efficiency value helps identify deterioration of the system's ability to heat or cool. In response to a predetermined amount of deterioration, a control signal indicates a need for inspection of the system before the system fails completely.
However, a disadvantage of current thermostats is their inability to quantify certain operating limits of heating or cooling systems. For example, conventional thermostats fail to predetermine the maximum outdoor temperature that a particular cooling system can tolerate while still maintaining the indoor temperature at its desired level. And thermostats typically fail to predetermine the minimum or maximum achievable indoor temperature for a given outdoor temperature.